The invention relates to a variable valve drive of an internal combustion engine for actuating a gas-exchange valve. Its motion follows a lift of a cam and also a lift of a piston of a hydraulic force-applying device superimposed on the lift of the cam and independent of the lift of the cam. This is connected to a hydraulic medium line with adjustable hydraulic medium pressure and has a pressure chamber charging the piston and also a hydraulic valve lash compensating device with a work chamber limited radially by a housing.
Valve drives according to this class are known in the state of the art. For example, DE 43 18 293 A1 discloses a hydraulic force-applying device on a finger lever drive with a pivot support, which pivotably supports a finger lever actuated by a cam in the actuation direction of the gas-exchange valve. The hydraulic force-applying device here expands the functionality of a hydraulic valve lash compensating device by a hydraulic lift, which is variably adjustable and which is superimposed on the mechanical lift given by the cam on the gas-exchange valve. Through this superimposition, on one hand, a reduction of the gas-exchange valve lift in terms of maximum lift and/or opening period up to complete standstill of the gas-exchange valve is possible. On the other hand, by superimposing the hydraulic and the mechanical lift, an expansion of the lift generated by the cam in the sense of an earlier opening time or a later closing time or an increased maximum lift or combinations of the like are possible.
For realizing this functionality, the publication noted above proposes an essentially conventional pivot support common for someone skilled in the art with hydraulic valve lash compensation. This pivot support is guided so that it can move longitudinally in an additional outer housing, which is supported in a recess of the internal combustion engine. Here, a bottom side of the pivot support, together with an inner wall of the outer housing, comprises a pressure chamber, which is connected to a pressure-adjustable hydraulic medium line.
Although the use of a conventional pivot support, which is to be modified, if necessary, based on changed movement and installation relationships, promises acceptable production costs of the hydraulic force-applying device, on one hand the increased installation requirements in terms of diameter and length are disadvantageous due to the addition of additional wall thickness. Such an extension of the diameter and length is to be viewed as critical for modern installation-limited internal combustion engines, because the wall thickness between the receptacle bore is already small for supporting the pivot support and adjacent hollow spaces, for example, charge changing and cooling water channels or spark plug shafts, and permit less play for an extension of the diameter or length of the receptacle bore.
Another disadvantageous aspect given with the use of such a pivot support is the increase in the mass of the moving valve drive components. Thus, it is necessary for a hydraulic force-applying device according to the cited publication that the pivot support must be incorporated completely in the hydraulic lift that is sometimes characterized by high acceleration values. The mass of the pivot support moved at the same time thus requires either a limitation of these acceleration values to values that worsen the quality of the charge change or a high hydraulic drive output is necessary for achieving high acceleration values of the hydraulic lift. The latter must be applied, however, in a direct or indirect way by the internal combustion engine itself and is to be limited to a minimum with respect to a tolerable increase in the frictional output of the internal combustion engine.
The described disadvantages incidentally do not apply just for the cited finger lever drive, but instead also for other valve drive constructions. This applies to a greater degree for valve drives, in which the components of the hydraulic force-applying device also follow the mechanical lift of the cam, as is the case, for example, for cup-tappet drives. In this respect, an increase in the moving mass would have a disadvantageous effect, in particular, on the achievable acceleration values of the valve drive.